1. Field of the Invention
The method and the apparatus which are the object of the invention relate to the use of eddy currents for detecting faults and discriminating said faults in severity in a passing series of metal tubes.
2. Discussion of the Background
Means of monitoring defects in metal tubes and which resort to eddy currents are already known. Such means comprises a solenoid which is supplied with a current of relatively high frequency which generates an alternating magnetic field. This alternating magnetic field in turn generates eddy currents in the wall of the tube, which interact with the faults present in the form of discontinuities in the metal or alloy to be inspected. These interactions are detected by means of at least one receiving solenoid of which the fluctuations in impedance as the faults pass by are measured. The use of a receiving solenoid comprising two opposing windings makes it possible to enhance the sensitivity of measurement.
In the case of magnetic metals such as carbon steels, these same inspecting means may be used on condition that the metal is saturated with a sufficiently intense continuous magnetic field for its permeability to be reduced to a constant level for a given product and comparable with that of a non-magnetic metal or alloy. In the case of a carbon steel, this permeability is thus virtually brought to the level of that of an austenitic steel.
Thus it is possible to detect faults of relatively small dimensions such as fissures or breaks in cohesion of greater or lesser depth and which may even pass right through the metal and be the source of leakage. Such an inspection is performed continuously by causing the tubes to travel through the solenoids of the detection apparatus which employs eddy currents, at speeds which may be as much as several meters per second.
Experience shows that these inspecting means do not make it possible reliably to identify from all the defects detected, those which are capable of causing leakages through the wall of a tube. Indeed, a surface fault may give a signal the amplitude of which may be equal to or greater than that of a signal corresponding to a fault of relatively considerable depth. Well, generally speaking, tubes having superficial faults may be recovered by some secondary operation or repair while other tubes having deep faults have to be rejected.
For these reasons, it is in many cases necessary to verify the extent of the faults detected by eddy currents by using some other method such as a visual inspection or a hydraulic test for sealing-tightness.
Such hydraulic tests make it possible to establish that only a small proportion of the tubes rejected during the course of eddy current inspections, on account of open faults producing a signal of an amplitude corresponding to that of a standard fault, do actually exhibit leakages under internal fluid pressure.
Attempts have been made to find ways of using eddy current inspection means which make it possible to select from all the faults detected on the walls of metal tubes of all types said faults being either internal faults or faults opening on the outside surface of the tubes those faults which are of relatively substantial depth and distinguish them from superficial faults. It may also happen that the faults or defects open on the inside surface of the tubes, the faults or defects being either internal or opening also on the outside surface of the tubes (going through defects).
In particular, a possible method of determining a rejection threshold has been sought which might with virtual certainty guarantee the rejection of tubes exhibiting faults which pass almost completely through the thickness while not at the same time rejecting a considerable quantity of tubes which suffer from faults opening on the outside surface of the tube of a depth which does not exceed about 10% of the thickness of the tube. Finally, attempts have been made to find a way of carrying out such inspections on tubes having an outside diameter which may be as much as or greater than 200 mm while their thickness may be at least 10% of their diameter, such tubes passing by the inspection means at speeds of several meters per second.